1. Field of Invention
The present invention relates to a system and method for holographic storage, and more particularly to a system and method for holographic storage using a set of reflectors to split a light source.
2. Related Art
At present, in the optical storage medium market, the capacity of a commercialized blue-ray disc cannot be larger than 100 GBytes, so various possible super capacity recording techniques are being developed in a wide range. Among these techniques, the holographic optical disc is the one with the most potential. Though the holographic recording technique has been developed for a long time, it cannot be applied to consumer optical storage commodities for various reasons. For example, all of the early holographic experiments required huge high-power laser light sources of hundreds of milliwatts and complicated optical systems, and a heavy vibration-free table. Moreover, the photo-refractive crystal usually used as the holographic recording medium is more expensive than other, more average-priced media. However, along with technical progress and conceptual changes, the limitations of the holographic storage recording technique have been eliminated one after another. For instance, miniaturized high-power lasers, high photosensitive recording materials, and miniature data access optical systems have been substantiality developed. Besides, the belief that a recording medium must be rewritable has wavered due to consumer behavior in the optical disc market. Till now, it was difficult to make the recording material of the rewritable photo-refractive crystal meet the requirements of favorable material property, high data safety, and low price. However, enlightened by the wide popularization of average-priced optical discs such as the write-once CD-R/DVD-R, holographic recording without using rewritable media materials has begun to be widely accepted. When not considering rewritability, many cheap organic materials of high photosensibility, for example: photopolymer, can be selected as the data-recording layer of the holographic optical disc. When the photopolymer is irradiated by strong recording light, a chemical reaction such as the linkage of molecules occurs. Therefore, recording of three-dimensional holographic interference fringes of data and data reproduction can be performed via changes in optical properties caused by manipulating the density of the molecule linkage. The aforementioned concept of miniature data access optical system having position-servo function comes from the servo mechanism of CD/DVD optical disc drives, which is the key point in using holographic optical discs.
U.S. Patent Gazette Publications No. 20040212859 and U.S. Pat. No. 6,700,686 disclose the application of the holographic storage technique in a transmissive holographic recording medium. As it is a transmissive design, the image sensor is disposed at the other side of the holographic recording medium. As such, the overall volume of the system is enlarged. Besides, the transmissive system architecture design usually has the optical axis of the object lens passing the signal beam perpendicular to the holographic recording medium, and the reference beam must be obliquely irradiated to the holographic recording medium. Therefore, the relative position and direction between the reference beam and the holographic recording medium deviate easily. Once the deviation occurs, the reproduced signal beam will not be formed if the reference beam cannot be irradiated to the holographic recording medium along the original path. Thus, the produced signal beam cannot be captured by adjusting the signal beam path. Therefore, the image sensor for receiving produced light signals cannot receive any reproduced beam signals, and, of course, it cannot recover the correct reproduced data by an image processing technique. Though for a static holographic recording medium, the reproduced beam signal can be obtained if the architecture allows for scanning the position and direction of reference beams in a small range. However, as for a continuously moving holographic recording medium, it is still hard to achieve the same purpose of obtaining the reproduced signal.
Other relative arts disclosed in U.S. Pat. No. 6,721,076 and U.S. Pat. No. 6,909,529 provide an optical architecture for reflective holographic recording medium. However, no specific servo methods are provided.
In addition, U.S. Pat. No. 6,909,529 provides an optical architecture for a reflective holographic recording medium with servo. However, as the light beams are all focused by lenses as convergent elements, wavelength multiplexing cannot be applied.